Processes for the preparation of diphenyl sulfone compounds of Formula (II) are disclosed in Japanese Patents Laid-open Nos. Sho 58-20493, Sho 58-82788, Sho 60-13852 and Sho 60-56949, that 4,4′-dihydroxydiophenyl sulfone is reacted with a halogen compound, such as alkyl halide, in a polar solvent, such as dimethylformamide or alcohol, in the presence of an alkali. These processes have a drawback of difficulty to improve the selectivity of reactions because of the use of solvents that dissolve reaction reagents and products well. There was a problem that noticeable amounts of diether derivatives, i.e., 4,4′-disubstituted hydroxydiphenyl sulfones, are produced as byproducts.
WO 91/11433 has reported that a reaction of a mole of 4,4′-dihydroxydiphenyl sulfone with an alkyl halide in the presence of 1.5 to 3 moles of alkali in 0.3 to 1.5 parts by weight of an aqueous solvent to a part by weight of 4,4′-dihydroxydiphenyl sulfone results in satisfactory selectivity and yield. However, when alkyl chlorides are used, it takes a long time to complete the reaction because of their low reactivity. Besides, alkyl iodides are expensive. Therefore, alkyl halides able to use in industry are practically limited to alkyl bromides. The said WO patent reports only examples of using alkyl bromides.
If an alkylation is carried out using an alkyl bromide under the above reaction conditions, crystals are deposited as the reaction proceeds. The inside of the system becomes viscous and heterogeneous, resulting in insufficient stirring. Therefore, there is a problem of incompletion of the reaction.
Of compounds of Formula (II) useful as developers for leuco dyes to be used in thermal recording papers, compounds where R is isopropyl are produced using isopropyl bromide, which is hardly produced domestically. Therefore it is difficult to procure the starting material stably.
It is an object of the present invention to provide industrial processes for the production of diphenyl sulfone compounds of Formula (II) by using alkylating agents that are supplied stably, instead of alkyl bromide, with productivity, economical efficiency and safety.